1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and an apparatus for radio connection setup in order to achieve efficient radio resource management in a mobile communication system.
2. Description of the Related Art
In general, wireless communication systems can be classified according to their multiplexing schemes, which include a time division multiplexing scheme, a code division multiplexing scheme, an orthogonal frequency multiplexing scheme, etc. Among those schemes, the code division multiplexing scheme is now most widely used and can be divided into a synchronous scheme and an a synchronous scheme. Because the code division multiplexing scheme uses codes, the code division multiplexing scheme is now short of resources due to the lack of orthogonal codes. Therefore, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is now drawing a lot of attention.
The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers (sub-carrier channels) before being transmitted. The OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme. However, the OFDM scheme transmits multiple sub-carriers while maintaining the orthogonality between them, and uses the frequency spectrum in an overlapping manner. Therefore, the OFDM scheme is more efficient in its use of frequency resources, is more robust against frequency selective fading, and can reduce inter-symbol interference (ISI) by using guard intervals. Further, the OFDM scheme enables design of an equalizer having a simple hardware structure, and is robust against impulse noise. Therefore, the OFDM scheme can achieve an optimum transmission efficiency for high speed data transmission.
In the wireless communication system as described above, degradation in the quality of a high quality data service is caused mainly by the channel environment. The channel environment in the wireless communication system frequently changes due to interference by multi-path signals or other users, Doppler Effect movement and frequency speed change of a User Equipment (UE), shadowing, change in the power of a received signal caused by fading as well as Additive White Gaussian Noise (AWGN), etc. Therefore, in order to support a high quality data service in the wireless communication, it is necessary to effectively overcome such degradation factors.
One of the main schemes used in order to overcome fading in a typical OFDM system is the Adaptive Modulation and Coding (AMC) scheme. According to the AMC scheme, the modulation scheme and the coding scheme are adaptively controlled according to a channel change in a downlink (DL). Usually, it is possible to detect Channel Quality Information (CQI) of the downlink by measuring a Signal to Noise Ratio (SNR) of a received signal in a UE. The UE feedbacks the channel quality information of the downlink to a network through an uplink (UL).
The network estimates a channel state of the downlink based on the channel quality information of the downlink fed back from the UE, and determines a modulation scheme and a coding scheme in accordance with the estimated channel state. According to the AMC technology, a high-order modulation scheme and a high coding rate are applied in a good channel state, while a low-order modulation scheme and a low coding rate are applied in a bad channel state. In comparison with the conventional schemes relying on high speed power control, the AMC scheme can improve the average capability of a system by enhancing the system's capability of adapting itself to temporally changeable characteristics of a channel.
FIGS. 1A and 1B are a block diagram illustrating a structure of a typical 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, which is a next generation mobile communication system capable of replacing the Universal Mobile Telecommunication System (UMTS), and which is a standard for the 3rd generation mobile communication currently being discussed in the 3GPP.
Referring to FIG 1A, the 3GPP LTE system includes a UE 11 for an LTE system and an Evolved Radio Access Network (E-RAN) 14, which performs functions of both a node B and a Radio Network Controller (RNC) in an existing 3GPP system. In an existing 3GPP system, a node B is a radio network apparatus which performs by itself communication with UEs and controls a cell, and an RNC controls multiple node Bs and radio resources. In the E-RAN 14, as is in the existing 3GPP system, functions of an Evolved Node B (E-NB) 12 and an Evolved RNC (E-RNC) 13 may be either physically separately distributed to different nodes or merged within one node.
For convenience of description, the following description is based on an example in which the E-NB 12 and the E-RNC 13 have been merged within one node. However, the present invention naturally includes the case in which the E-NB 12 and the E-RNC 13 are physically distributed separately to different nodes.
The E-CN 15 may be a node in which functions of a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) are merged. The E-CN 15 is located between a Packet Data Network (PDN) 16 and the E-RAN 14, allocates an IP address to the UE 11, and serves as a gateway connecting the UE 110 to a packet data network (PDN) 16. The definition and functions of the SGSN and the GGSN are based on the standard of the 3GPP and will not be described in more detail here.
Referring to FIG. 1B an Evolved UMTS Radio Access Network (E-RAN) 110 has a simple 2-node structure including Evolved Node Bs (ENBs) 120, 122, 124, 126, and 128 and anchor nodes 130 and 132. A User Equipment (UE) 101 is connected to an Internet Protocol (IP) network through the E-RAN 110. The ENBs 120 to 128 correspond to existing Node Bs of the UMTS system and are connected with the UE 101 through radio channels. Differently from the existing Node Bs, the ENBs 120 to 128 perform more complicated functions. In the LTE system, all user traffic, including a real time service such as Voice over IP (VoIP) through an Internet Protocol, are provided through shared channels. Therefore, the LTE system requires units for collecting and scheduling state information of the UEs. Such operations are performed by the ENBs 120 to 128.
Usually, one ENB controls a plurality of cells. Further, the ENB performs Adaptive Modulation and Coding (AMC) for determining a channel coding rate and a modulation scheme in accordance with the channel state of a UE. Further, as the Enhanced Dedicated Channel (E-DCH), High Speed Uplink Packet Access (HSUPA), and High Speed Downlink Packet Access (HSDPA) of the UMTS, Hybrid ARQ (HARQ) is performed between the ENBs 120 to 128 and the UE 101 in the LTE also. However, only HARQ is insufficient in order to satisfy the requirement for diverse Qualities of Service (QoSs). Therefore, separate ARQ (outer ARQ) in a higher layer may be performed between the UE 101 and the ENBs 120 to 128.
HARQ is a transmission scheme for improving a ratio of success in packet reception by soft-combining previously-received data with retransmitted data without discarding the previously-received data. HARQ is used in order to improve the transmission efficiency in high speed packet communication such as High Speed Downlink Packet Access (HSDPA), Enhanced Dedicated Channel (EDCH), etc. It is expected that, in order to achieve a transmission speed of maximum 100 Mbps, the LTE should use the Orthogonal Frequency Division Multiplexing (OFDM) as wireless access technology in the 20 MHz bandwidth.
Current 3GPP standard organizations are discussing statuses or modes of UEs in a 3GPP LTE system, which can be classified into an RRC idle mode and an RRC connected mode. The RRC is a layer located on a control plane of the E-RAN and the UE, which transmits/receives radio access-related control information through the RRC layer. The RRC idle mode refers to a mode of the UE, in which the E-RAN does not have RRC context information about the UE and there exists no control channel (RRC connection) between the UE and the E-RAN. In contrast, the RRC connected mode refers to a mode of the UE, in which a control channel (RRC connection) exists between the UE and the E-RAN and the E-RAN has RRC context information about the UE.
FIG. 2 is a signal flow diagram illustrating an example of a conventional RRC connection establishing procedure and procedures that can be performed thereafter by a UE.
Referring to FIG. 2, a UE 201 is initially in an RRC idle mode in step 211. When there is a request for a signaling connection to the E-RAN 202 or an E-CN (not shown) from a higher layer in the RRC idle mode, an RRC connection is established between the UE 201 and the E-RAN 202 through the RRC connection setup process of steps 221 to 223. A control channel through which RRC control information, etc. can be transmitted is established between the UE 201 and the E-RAN 202, and the E-RAN 202 can maintain/manage a context for the UE 201.
In step 221, the UE 201 transmits an RRC connection request message requesting the RRC connection. In step 222, in response to the RRC connection request message of step 221, the E-RAN 202 transmits an RRC connection setup message including control channel information for the UE 201. The RRC connection setup message may include resource allocation information for reporting CQI to be performed by the UE, measurement control information, uplink timing sync procedure information, etc.
The resource allocation information for reporting CQI may include radio resource information in the time and frequency domain, a start point, a period, etc. The measurement control information may include a list of neighbor cells and parameters requiring measurement within frequency/between frequencies/between systems, gap generation information for measurement between frequencies/between systems, etc. The uplink timing sync procedure information may include a period of the uplink timing synchronization to be performed, etc. The information as described above may either be included in the transmitted RRC connection setup message as in step 222 or be transmitted through separate signaling as in steps 231 to 233. The E-RAN 202 may transmit resource allocation information for reporting CQI to the UE 201 as in step 231, or may transmit measurement control information to the UE 201 as in step 232, or may transmit uplink timing sync procedure information to the UE 201 as in step 233. The sequence in which the information is transmitted may be changed according to the way in which the present invention is implemented.
The RRC connection setup completion message transmitted in step 223 is transmitted from the UE 201 to the E-RAN 202 in order to notify that the RRC connection has been successfully competed in response to the RRC connection setup message of step 222.
The CQI reports transmitted by the UE 201 in steps 241 to 246 include the RRC connection setup message of step 222 or the resource allocation information of step 231 for reporting CQI to the UE 201, and are performed based on resources/intervals for the CQI report received through separate signaling. Through the CQI report of steps 241 to 246, the E-RAN 202 recognizes a current channel state of the UE 201 and sets an AMC level for data transmission based on the CQI report.
The uplink timing sync procedure of steps 251 and 252 includes parameter information for the uplink timing synchronization procedure of step 233 or the RRC connection setup message of step 222, and performs a periodic uplink timing sync procedure according to a period of the uplink timing synchronization procedure received through separate signaling. The uplink timing synchronization procedure of steps 251 and 252 is performed in order to acquire synchronization between a time point for uplink transmission by the UE 201 and a time point for uplink reception by the E-RAN 202 from the UE 201.
In step 261, a gap interval is generated based on gap information for measuring neighbor cells between frequencies/between systems received through separate signaling, which includes the measurement control information of step 232 or the RRC connection setup message received in step 222.
In the gap interval generated in step 261, the UE 201 interrupts channel reception from a current cell of a current frequency band and performs measurement for neighbor cells of another system or another frequency band indicated by measurement control information, and the E-RAN 202 does not perform transmission to the UE 201. The UE 201 continues to perform measurement for neighbor cells within the frequency, except for the gap interval as in step 261, based on the list of neighbor cells within the frequency received through the measurement control information.
As noted from FIG. 2, according to the prior art, the UE 201 performs, through a pre-established gap interval, measurement for neighbor cells between frequencies/between systems and measurement within frequency/between frequencies/between systems after shifting to an RRC conriected mode. Therefore, the UE 201 must perform many complicated procedures, such as a periodic CQI report using radio resources, a periodic uplink timing sync procedure using radio resources, etc., which cause many problems including excessive power consumption, complexity, etc. Further, the E-RAN 202 allocates many radio resources in order to allow the UEs to perform the measurement, CQI report, uplink timing sync procedure, etc., which result in inefficient use of radio resources.